Concrete and asphalt have been commonly used as lining materials for preventing water leakage from reservoirs, but recently, it is more common to use vulcanized rubber sheeting that withstands large deformations, has high weatherability, and which is flexible enough to absorb the subsequent movement of the lined area (e.g., river bed). But the nature of the process for manufacturing vulcanized rubber sheeting imposes dimensional limitations on the production equipment, and although sheeting as long as about 200 m can be made without great difficulty, its width is usually about 1.0 m, and making sheeting wider than 10 m is practically impossible. Therefore, the current practice is to transport vulcanized rubber sheets to the installation site, where they are then joined together.
Several methods have been proposed to make the overlapped areas of the joined sheets resistant against water leakage. The two most common methods are to use an adhesive agent, and to use an unvulcanized rubber tape. However, neither method is completely satisfactory. In the first method, bonding had a tendency to become uneven because a uniform film of the adhesive cannot easily be obtained. In addition, the adhesion strength varies greatly with the length of open time. The open time is alternatively called "open assembly time" and means the time which gives an appropriate tackiness to an adhesive by coating an adhesive to a material and allowing to stand the coated surface thereof to volatilize a solvent contained in the adhesive. The greater defect of this method is that no matter how good the quality of the vulcanized sheets and adhesive tape are, poor adhesion may result unless the persons applying the tape are very skilled. If a strong wind blows during open time, dust sticks on the surface and prevents strong adhesion. If the temperature is high, even a short open time will result in overdrying of the adhesive to cause poor adhesion. If the humidity is high, dew is formed on the joining surfaces, also causing poor adhesion. If the temperature is too low, a long open time reduces the joining efficiency. Thus, the sheet-joining workers must have adequate knowledge on the various environments under which the work may be executed. Therefore, with the first method, it is very difficult to join the waterproof vulcanized rubber sheets securely enough to ensure that there will be no water leaks.
An example of the second method that uses an unvulcanized rubber tape comprises sandwiching the unvulcanized rubber tape between vulcanized rubber sheets and spot-vulcanizing them by applying heat (150.degree.-200.degree. C.) and pressure (100-150 kg/cm.sup.2) to both obverse and reverse sides of the sheets. But this method requires a vulcanizing press on the job site. In addition, the desired bond strength is not obtained unless high vulcanization temperature and pressure are used, and this results in prolonged joining time, and reduced joining efficiency.
The conventional unvulcanized rubber tape has Mooney viscosity [ML.sub.1+5 (100.degree. C.)] of 23 and comprises the following compositions (parts by weight).
______________________________________ EPT (third component: dicyclopenta- 70 diene, iodine value: 20) IIR 30 ZnO 5 SRF carbon 40 Escoretz (product of Esso Chemical Co.) 15 Paraffinic process oil (product of 20 Maruzen Oil Co.) S 1.5 Dipentamethylenehexasulfide 5.5 ______________________________________